Neonates are most vulnerable as they begin their extrauterine life. During this time, some neonates need ventilatory support. Because of concerns about oxygen toxicity, however, neonates are aggressively weaned from the ventilator. Weaning puts neonates at greater risk of hypoxemia, particularly due to their high fetal hemoglobin level, which is associated with lower oxygen content in the tissues. Thus, ventilator weaning must follow a fine line between oxygen toxicity and oxygen insufficiency. During weaning, neonates are more sensitive to stimuli and frequently develop desaturation episodes, apnea, and bradycardia. During these times, accurate information about oxygen saturation (502) is crucial. Current routine bedside monitoring includes a pulse oximeter to measure peripheral oxygen saturation (SpO2) and a cardiac monitor to indicate heart rate. However, the pulse oximeter may not be accurate, and can affect clinicians' judgment when they wean neonates from the ventilator, which will affect the infant's oxygenation status. During ventilatory support, umbilical arterial and venous catheters are inserted routinely. Fiberoptic catheters can be used to monitor arterial (SvO and venous oxygen saturation (SvO. Monitoring of SvO2 in addition to SaO2 ma)? reflect more accurately the status of oxygen supply and demand in neonates. Concurrent measurement of human vagal tone, using the R-R intervals of the electrocardiogram (ECG) from a cardiac monitor, may also facilitate more accurate interpretation of oxygenation values, because decreased human vagal tone precedes apnea and bradycardia in preterm infants. Its relationship with oxygenation, however, is not documented. Thus, the aims of this clinical trial are to: I. examine the effects of using fiberoptic umbilical catheters to monitor SaO2 and SvO2 on ventilator weaning of neonates and the number of oxygenation however, is not documented. Thus, the aims of this clinical trial are to: a. the effect of fetal hemoglobin on the accuracy of 502 monitoring, using a gold standard co-oximeter; and b.the association between 502 and oxygen tension values in neonates 3. examine the effects of ventilator weaning on 502 readings and vagal tone.